A box (also called block or region) is a 3x3 sub-grid that must contain the digits 1 through 9 exactly once.
A box in Sudoku is a 3x3 group of nine cells outlined by thicker borders within the 9x9 grid. A standard Sudoku puzzle contains exactly nine boxes, arranged in three rows and three columns. Each box must contain every digit from 1 through 9 without repetition.
The term "box" is the most commonly used name, but you will also see it called a block, region, square, or nonet depending on the source. All of these terms refer to the same 3x3 sub-grid. The box constraint is what gives Sudoku its unique character and distinguishes it from simpler Latin square puzzles.
In variant puzzles like Jigsaw Sudoku, the regions are irregularly shaped rather than square, but the same constraint applies: each region must contain all digits exactly once. For a full overview of Sudoku structure, see our Sudoku rules guide.
Every cell in a Sudoku puzzle belongs to exactly three constraint groups: a row, a column, and a box. The box constraint is what makes Sudoku challenging. Without it, you would only need to ensure digits are unique across rows and columns, which is a much simpler problem.
When solving, the box constraint lets you eliminate candidates that already appear elsewhere in the same box. For example, if a box already contains the digits 1, 3, 5, and 7, then every empty cell in that box can only contain 2, 4, 6, 8, or 9. Combined with row and column constraints, this often narrows a cell down to a single possibility.
Several important solving strategies are built around the box constraint. Crosshatching, the most basic technique, involves scanning rows and columns to determine where a digit can go within a specific box. This is often the first technique beginners learn.
More advanced box-based strategies include pointing pairs, where a candidate in a box is limited to a single row or column, allowing eliminations outside the box. Similarly, box-line reduction uses the interaction between boxes and lines to eliminate candidates within a box.
Understanding how boxes interact with rows and columns is fundamental to progressing beyond beginner-level solving. Return to the Sudoku glossary to explore more terms and concepts.
A standard 9x9 Sudoku puzzle contains exactly nine boxes, each measuring 3x3 cells. The boxes are arranged in a 3x3 pattern across the grid, giving the puzzle its distinctive visual structure.
Box, block, and region all refer to the same thing in standard Sudoku: a 3x3 sub-grid. Different books, apps, and communities use different terms, but they all describe the same constraint group of nine cells.
Boxes provide one of the three fundamental constraints in Sudoku. Each box must contain every digit from 1 to 9 exactly once. Without the box constraint, Sudoku would be a much simpler puzzle to solve.
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